AudioStretchy

AudioStretchy is a Python library and CLI tool that which performs fast, high-quality time-stretching of WAV/MP3 files without changing their pitch. Works well for speech, can time-stretch silence separately. The library is a wrapper around David Bryant’s audio-stretch C library.

Version: 1.2.1

Features

• Time stretching of audio files without changing their pitch
• Supports WAV files, and optionally MP3 files
• Adjustable stretching ratio from 0.25 to 4.0
• Cross-platform: Windows, macOS, and Linux
• Optional resampling

Time-domain harmonic scaling (TDHS) is a method for time-scale modification of speech (or other audio signals), allowing the apparent rate of speech articulation to be changed without affecting the pitch-contour and the time-evolution of the formant structure. TDHS differs from other time-scale modification algorithms in that time-scaling operations are performed in the time domain (not the frequency domain).

The core functionality of this package is provided by David Bryant’s excellent audio-stretch C library that performs fast, high-quality TDHS on WAV in the ratio range of 0.25 (4× slower) to 4.0 (4× faster).

The library gives very good results with speech recordings, especially with modest stretching at the ratio between 0.9 (10% slower) and 1.1 (10% faster). AudioStretchy is a Python wrapper around that library. The Python package also offers some additional, optional functionality: supports MP3 (in addition to WAV), and allows you to preform resampling.

Installation

Simple installation

To be able to stretch and resample both WAV and MP3 files, install AudioStretchy using pip like so:

pip install audiostretchy[all]

Efficient installation

To only be able to stretch WAV files (no resampling, no MP3 support), install AudioStretchy with minimal dependencies like so:

pip install audiostretchy

Development installation

To install the development version, use:

python3 -m pip install git+https://github.com/twardoch/audiostretchy#egg=audiostretchy[all]

Usage

CLI

audiostretchy INPUT_WAV OUTPUT_WAV <flags>

POSITIONAL ARGUMENTS
    INPUT_PATH
        The path to the input WAV or MP3 audio file.
    OUTPUT_PATH
        The path to save the stretched WAV or MP3 audio file.

FLAGS
    -r, --ratio=RATIO
        The stretch ratio, where values greater than 1.0 will extend the audio and values less than 1.0 will shorten the audio. Default is 1.0 = no stretching.
    -g, --gap_ratio=GAP_RATIO
        The stretch ratio for gaps (silence) in the audio. Default is 0.0 = use ratio.
    -u, --upper_freq=UPPER_FREQ
        The upper frequency limit for period detection in Hz. Default is 333 Hz.
    -l, --lower_freq=LOWER_FREQ
        The lower frequency limit. Default is 55 Hz.
    -b, --buffer_ms=BUFFER_MS
        The buffer size in milliseconds for processing the audio in chunks. Default is 25 ms.
    -t, --threshold_gap_db=THRESHOLD_GAP_DB
        The threshold level in dB to determine if a section of audio is considered a gap (silence). Default is -40 dB.
    -d, --dual_force=DUAL_FORCE
        If set, forces the algorithm to operate in dual-force mode, which may improve the quality of the stretched audio but may also increase processing time.
    -f, --fast_detection=FAST_DETECTION
        If set, enables fast period detection, which may speed up processing but reduce the quality of the stretched audio.
    -n, --normal_detection=NORMAL_DETECTION
        If set, forces the algorithm to use normal period detection instead of fast period detection.
    -s, --sample_rate=SAMPLE_RATE
        The target sample rate for resampling the stretched audio in Hz. Default is 0 = use sample rate of the input audio.

Python

from audiostretchy.stretch import stretch_audio

stretch_audio("input.wav", "output.wav", ratio=1.1)

In this example, the input.wav file will be time-stretched by a factor of 1.1, meaning it will be 10% longer, and the result will be saved in the output.wav file.

For advanced usage, you can use the AudioStretch class that lets you open and save files provided as paths or as file-like BytesIO objects:

from audiostretchy.stretch import AudioStretch

audio_stretch = AudioStretch()
audio_stretch.open(file=MP3DataAsBytesIO, format="mp3")
audio_stretch.stretch(
    ratio=1.1,
    gap_ratio=1.2,
    upper_freq=333,
    lower_freq=55,
    buffer_ms=25,
    threshold_gap_db=-40,
    dual_force=False,
    fast_detection=False,
    normal_detection=False,
)
audio_stretch.resample(sample_rate=44100)
audio_stretch.save(file=WAVDataAsBytesIO, format="wav")

License

• Original C library code: Copyright (c) 2022 David Bryant
• Python code: Copyright (c) 2023 Adam Twardoch
• Written with assistance from GPT-4
• Licensed under the BSD-3-Clause license